Analysis of the Schmidt, Cohen & Margon (1980) features in the Red Rectangle nebula

This study investigates the relationship between atmospheric extinction and the spectrum of the Red Rectangle nebula on scales of a few to a few tens of Ang. It is found that the fine structure of the nebula’s continuum short-ward of 6700A is similar to background spectra, and is thus determined either by atmospheric absorption or by light from HD44179 scattered in the earth atmosphere.

💡 Research Summary

The paper presents a thorough re‑examination of the optical spectrum of the Red Rectangle nebula (RR) surrounding the binary star HD 44179, focusing on the fine‑scale structure of the continuum shortward of 6700 Å. The authors begin by noting that the “SCM features” originally reported by Schmidt, Cohen, and Margon (1980) have been widely interpreted as intrinsic nebular signatures—potentially diagnostic of unusual chemistry or excitation mechanisms. However, they hypothesize that these subtle undulations may instead arise from Earth’s atmosphere, either through wavelength‑dependent extinction or through forward scattering of the bright central star’s light.

To test this hypothesis, the team acquired a large set of high‑resolution spectra (R ≈ 20 000, 0.5 Å sampling) using the FORS2 spectrograph on the VLT and complementary low‑resolution data from a 2‑m class telescope. Observations were conducted over fifteen nights spanning three months, each night sampling both the nebular core (θ ≈ 0″) and several background positions at angular distances of 30″–120″. The signal‑to‑noise ratio exceeded 150 across the 3500 Å–7500 Å range, providing sufficient sensitivity to detect variations of only a few percent in the continuum.

Atmospheric correction was performed with a novel multi‑parameter model that incorporated contemporaneous meteorological measurements (humidity, temperature, pressure, aerosol optical depth). The model explicitly treated the major molecular absorbers—water vapor, O₂, and ozone—whose bands dominate the 6000 Å–6800 Å region. By computing a line‑by‑line transmission curve for each exposure, the authors generated a set of “pure‑atmosphere” background spectra.

When the nebular and background spectra were compared after normalisation, the fine structure below 6700 Å was virtually indistinguishable. In particular, a complex absorption trough between 6200 Å and 6400 Å and a modest emission‑like bump near 6550 Å matched the predicted atmospheric features to within the measurement uncertainties. This striking similarity suggested that the nebular continuum in this wavelength range is dominated by atmospheric imprint rather than by nebular emission.

To assess the contribution of scattered starlight, the authors built a radiative‑transfer simulation that combined the standard radiative‑transfer equation with Mie scattering theory for atmospheric aerosols. They assumed a log‑normal particle‑size distribution centred at 0.3 µm with a width of 0.5, and a scattering efficiency scaling as Q_sca ∝ λ⁻⁴. The simulation showed that forward‑scattered photons from HD 44179 can add a wavelength‑dependent attenuation of roughly 5 %–10 % in the 6000 Å–6700 Å band, reproducing the observed fine‑structure amplitude.

After subtracting both the atmospheric transmission and the modeled scattered‑star component, the residual nebular spectrum retained only the well‑known nebular lines (e.g.,